The University of Nottingham
Postgraduate Division of Nursing
Master of Nursing
Biological Foundations for Health
Worksheet 5: Reflexes
Answer Guide
Reflexes
Learning Outcomes
1. Define the terms ‘monosynaptic reflex’, ‘polysynaptic reflex’ and reciprocal inhibition
2. Draw a diagram describing the components and operation of the stretch reflex eg knee jerk
3. Explain using an everyday example the flexor (withdrawal) reflex eg nociceptive reaction and the
crossed extensor reflex
4. State the clinical importance of testing reflexes and list those commonly used in neurological assessment and list some conditions that show altered reflex responses
5. List 3 autonomic (visceral) reflexes
Stretch Reflex
Read the section on the stretch reflex activity (Marieb pages 484-488).
Reflexes are fast responses to stimuli produced by the internal or external environment. They are important control mechanisms for maintaining homeostasis. A spinal reflex is one that only involves the spinal cord. Cranial reflexes involve the brain centres and cranial nerves while those that cause the contraction of smooth or cardiac muscle or secretion by glands are autonomic reflexes.
Draw a diagram of the stretch reflex, use Figure 13.15(a) in Marieb to help you
Exercise 4.5
1. The stretch reflex is an example of a monosynaptic reflex arc. Why is it so called?
The main monosynaptic reflex arc consists of two neurones, one sensory and one motor, separated by a single synapse.
2. What do the muscle spindles monitor?
The spindles monitor changes in the length of the muscle by responding to the rate and degree of change in length.
3. What is the name given to a reflex arc in which the sensory nerve impulse enters the spinal cord on the same side that the motor nerve impulse leaves the spinal cord?
Ipsilateral
4. List two functions of the stretch reflex?
Maintains muscle tone; prevents over-stretching of muscle during movements; basis for clinical tests eg knee jerk (patellar reflex).
Reflexes other than the stretch reflex operate to monitor the movement of muscles. The Golgi tendon receptors located in the muscle’s tendon ensure smooth onset and termination of muscle contraction particularly in activities involving rapid switching between flexion and extension, such as running.
What is meant by the term reciprocal inhibition?
Reciprocal inhibition is when action potentials stimulate contraction of one muscle and simultaneously inhibit contraction of antagonistic muscles.
Read the 2 paragraphs on the flexor reflex and the crossed extensor reflex (pages 488- 489)
Exercise 4.6
1. In a table or in a few sentences contrast the flexor reflex and the crossed-extensor reflex.
In the flexor reflex a pin, say, stimulates pain receptors in the skin, action potentials are produced which transmits the information to the spinal cord via the sensory neurone. In the spinal cord a second impulse is generated in an association neurone which generates a third impulse in the motor neurone. The motor neurone stimulates the flexor muscle and inhibits the extensor muscle thus withdrawing the limb from the painful stimulus. This is an ipsilateral reflex. In the crossed extensor reflex, as the name suggests extension occurs on the opposite side of the body to the flexor reflex.
2. What is meant by an intersegmental reflex arc and why is it important?
To withdraw the limb it is necessary to involve motor neurones at different segments of the spinal cord, ascending and descending association neurones ensure that the message is transmitted to upper and lower extremity muscles.
Other Reflexes of Clinical Significance
Reflexes are often used for diagnosing disorders of the nervous system and locating injured tissue. A weak or absent reflex gives an indication that a conduction pathway may be damaged. Sometimes a method called reinforcement is used whereby muscle groups other than those being tested are voluntarily tensed, this increases reflexes in other parts of the body. Four reflexes commonly tested in the clinical setting are the patellar reflex (knee jerk), achilles reflex (ankle jerk), Babinski sign and Plantar reflex.
Exercise 4.7
1. How are these reflexes elicited? In what conditions might these reflexes be impaired?
Patellar - elicited by tapping the patellar ligament; impaired in chronic diabetics, exaggerated in corticospinal injuries.
Achilles - elicited by tapping the Achilles tendon; absent in chronic diabetes, alcoholism, subarachnoid hemorrhages, exaggerated in cervical cord compression.
Babinski sign - gentle stimulation of outer margin of sole of foot produces extension of big toe with or without fanning of the other toes; occurs in children under 18 months of age; positive Babinski sign after 18 months abnormal due to lesion of corticospinal tract.
Plantar reflex - as above but refers to the normal response after 18 months - a curling of all the toe; tests integrity of L4-S2 regions and corticospinal tracts
Positive reflex proves:
(i) sensory and motor connections between muscle and spinal cord are intact
(ii) vigour of response indicates the degree of excitability of the spinal cord
Reflexes are:
Hypoactive – peripheral nerve damage or ventral horn injury
Absent – chronic diabetes mellitus, neurosyphilis, coma
Hyperactive – lesions of corticospinal tract reduces the inhibitory effect of the brain on the spinal cord eg polio, stroke patients
2. This worksheet has concentrated on somatic spinal reflexes involving skeletal muscle.
List three autonomic (visceral) reflexes. You will study these types of reflexes in other sections of the course.
Pupil reflex, baroreceptor reflex, swallowing reflex, vomiting reflex etc
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